Fusarium species associated with rhizosphere soil and diseased roots ...

CANADIAN JOURNAL OF PLANT PATHOLOGY 17 325-330, 1995

Fusarium species associated with rhizosphere soil and diseased roots of eastern white pine seedlings and associated nursery soil Cynthia M. Ocamb and Jennifer Juzwik U.S. Department of Agriculture, Forest Seniice, North Central Forest Experiment Station, 1992 Folwell Avenue, St. Paul, Mirzrzesotu 55108. Accepted for publication 1995 06 12 Fusarium species isolated from necrotic roots of eastern white pine (Pirzus sti-obus) seedlings in two nurseries included F. acuminatum, F. equiseti, F. oqsporum, F. oxysporum var. redolens, F. prol$eratum, F. sambucinum, F. solani, and F. sporotrichioides. In addition, all but F. sambuciauin were isolated from the rhizosphere; all, in addition to F: graminearilm, were also isolated from nonrhizosphere soil. Fusarium oxysporum, F. oxysporum var. redolens, and F. proliferatum were the most prevalent taxa in roots and nonrhizosphere soil. These three taxa plus F. solani predominated in rhizosphere soil. Species prevalence differed by site and date of collection, e.g. F. proliferatum was present at only one site. At least seven species of Fusarium were associated with seedling root rot and their prevalence differed according to site and time of year. Additiorzal key words: F. oxysporum. F. proliferatum, F. solani, Pinus strobus, root rot. Ocamb, C.M., and J. Juzwik. 1995. Fusarium species associated with rhizosphere soil and diseased roots of eastern white pine seedlings and associated nursery soil. Can. J. Plant Pathol. 17:325-330. DiffCrentes espttces de Fusariurn ont CtC isolCes des racines nCcrosCes de jeunes plants de pin blanc (Pinus strobus) dans deux pkpinikres. Parmi ces espkces, on a retrouvC le F: acuminatum, le F. equiseti, le F. oxysporum, le F. oxysporum var. redolens, le F. proliferatum, le F. sambucinum, le F. solani et le F. sporotrichioides. De plus, toutes ces espttces, sauf le F. sambucinurn, ont aussi Ctk isolCes de la rhizophkre; toutes ces espkces, en plus du F. graminearum, ont aussi CtC isolCes du sol non rhizosphkrique. Le F. oxysporum, le F. oxysporum var. redolens et le F. proliferatum ont kt6 les taxons les plus frkquemment retrouvks dans les racines et dans le sol non rhizosphkrique. Ces trois taxons, en plus du F. solani, prkdominaient dans le sol de la rhizosphkre. La predominance des espttces a variC avec les emplacements et les dates d'Cchantillonnage; par exemple, le F. proliferaturn a CtC retrouvk B un seul emplacement. Au moins sept espttces de Fusarium ont CtC associkes h la pourriture des racines des jeunes plants et leur predominance a varik selon les emplacements et les phiodes de l'annke. Mots clks additionels: F. oxysporum, F. proliferatum, F. solani, Pinus strobus, fonte des semis fusariose.

Fusarium root rot frequently causes severe losses of eastern white pine (Pinus strobus L.) seedlings in bareroot nurseries in the north central United States (Riffle & Strong 1960) and southern Ontario (Greifenhagen et al. 1990). In three surveys, mortality was 10-25% in white pine plots in Wisconsin State Forest Nurseries (Renlund 1980, Renlund 1981, Prey et al. 1983), and in another survey, up to 75% of the white pine seedlings had root rot (Prey et al. 1985). In 1986, 68% of the 571 000 white pine seedlings from one nursery were culled because of root rot (Prey et al. 1986). Fusarium oxysporurn Schlechtend.:Fr. has been isolated from root lesions on white pine seedlings in Wisconsin, Michigan, and Ontario nurseries (Riffle & Strong 1960, Enebak 1988, Honhart & Juzwik 1988). Fusarium moniliforlrle J. Sheld. and F. solani (Mart.) Sacc. have been isolated from diseased eastern white pine seedlings; however these observations were limited to a single isolate (Riffle & Strong 1960), or the Fusarium species causing root rot were not reported in reference to eastern white pine (Wenner & Merrill 1984). We conducted a study to identify and quantify Fusarium species associated with root lesions, rhizosphere soil, and nonrhizosphere soil of white pine seedlings. We wanted to determine if F. 0x3)sporum was present in necrotic white pine roots and

if other Fusarium sqecies were also associated with this disease.

Materials and methods Nursery fields. Operational fields in two nurseries were used as study sites: A14 at St. Williams Provincial Nursery, St. Williams, Ontario, Canada; and C50 at Wilson State Forest Nursery, Boscobel, Wisconsin, USA. The two nurseries are similar in latitude and species grown. Soils in both nurseries are loamy sands with 2-4% organic matter and 1.20 gfcrn2 average bulk density. The maximum water holding capacity was 20% in the Wisconsin field and 18% in the Ontario field; soil pH was 5.6 in the Wisconsin field and 5.4 in the Ontario field. Both fields have a history of root rot in white pine crops. One-half (130 m x 42 m) of the Ontario field was fumigated with dazomet (400 kg productfha) in August 1989; the other half was left unfumigated. Ten 130 m x 1.1 m bed-rows in both sections were seeded with stratified white pine in April 1990, and the field was maintained through each growing season according to standard nursery practices. One-half (168 m x 8 m) of the Wisconsin field was used in this study. This field was fumigated with methyl bromide-chloropicrin (MC-33, 393 kg product/ha) in August 1989, four 168 m x 1.1 m bed-rows were

326 CANADIAN JOURNAL OF PLANT PATHOLOGY, VOLU

seeded with white pine in October 1989, and the field was maintained through two growing seasons according to standard nursery practices. Field sampling. Twelve 1.1-m2 plots were established in each section of the Ontario field in April 1991, the beginning of the second growing season. Plots were placed according to a stratified random sample design with equal numbers of plots located in low, mid-slope, and high topographic areas of each section of the Ontario field. In the Wisconsin field, where little topographic variation was found, eight 1.1m2 plots were systematically located in each of thee blocks (56 m x 8 m) that were situated successively along the length of the field; representing the successive decrease in irrigation pipeline diameter down the length of the Wisconsin field. All plots included the entire width of the seedbeds. All samples were collected during the second growing season for each field, i.e. when seedlings were 2 years old. Sample dates differed by nursery: 30 April, 2 July and 2 October 1991 in the Ontario field; and 14 May, 21 July and 8 October 1991 in the Wisconsin field. Ten randomlyselected living seedlings were carefully removed from each plot on each date in each location. The seedlings were placed in polyethylene bags and stored at 5°C until processed (approximately 1 week later). Rhizosphere soil was collected from each seedling: excess soil was removed by gently shaking roots, and rhizosphere soil was collected on clean paper by gently rubbing each root by hand. The rhizosphere soils collected from 10 seedlings from each plot were combined into one sample per plot as the result of the low yield of soil. Rhizosphere soil was then air-dried and stored at 5°C until processed (approximately 2 weeks later). Cores of nonrhizosphere (or bulk) soil were collected with a 2-cm-diameter Oakfield tube sampler soil probe (Forestry Suppliers Inc., Jackson, MS 39284) to a depth of 15 cm, yielding approximately 40 cm3 of soil per core. Five soil cores were collected between seedling rows within each plot, combined by plot in a polyethylene bag, and stored at 5OC until processed (within 3 weeks). Isolation of Fusarium species. Rhizosphere and nonrhizosphere soils were assayed using serial dilution plating with 0.1% water agar. A 10-g subsample of the nonrhizosphere soil was assayed from each plot; soil moisture content for each sample was determined by oven drying an additional 10-g sample for 48 h at 105°C. Rhizosphere soil from each plot was assayed in a similar manner. One half mL of each dilution was spread on each of four plates of a medium selective for Fusarium species. Nash and Snyder's pentachloronitrobenzene-peptone agar (Nash & Snyder 1962) supplemented with aureomycin (Kommedahl et al. 1979) was used for all samples; this medium was amended with oxgall for

samples collected in Ontario (Papavizas 1967). Petri dishes were incubated for 7-14 days at 24OC under indirect lighting. Colonies of Fusarium species were transferred to acidified potato-dextrose agar (PDA) (Dhingra & Sinclair 1985) and incubated 12-20 days at 22°C under fluorescent lamps (three General Electric or Sylvania 40 W cool white tubes) supplemented with UV light (one Sylvania 40W tube, BLB series) with a 12-h photoperiod. Single-spore isolates were then obtained from each colony and stored on silica gel at 5°C (Windels et al. 1988). Seedling root systems were washed with tap water and examined for necrosis. Fungi were isolated from symptomatic roots by excising 1-cm-long root segments from the necrotic-healthy root tissue interface, immersing them in 0.5% NaOCl solution for 1-3 min. followed by a sterile distilled water rinse, and placing pieces of the excised segments on the previously described Fusarium-selective medium. The petri dishes were held for 7-14 days at 22-24°C under indirect lighting. As before, colonies of Fusarium were transferred to acidified PDA; singlespore isolates of each colony were collected, and isolates were stored on silica gel. Identification of Fusarium species. Identification of Fusarium to species was made by placing silica gel crystals on PDA and carnation leaf agar (Nelson et al. 1983). Cultures were incubated under the light and temperature regimes previously described. Each isolate was examined microscopically and identified to species according to the system of Nelson, Toussoun, and Marasas (1983).

Results Fusarium species recovered. When root lesion samples are compared, the greatest number of Fusarium taxa were isolated from root lesions on eastern white pine grown in the Wisconsin nursery whereas the fewest taxa were isolated from root lesions of trees grown in the nonfumigated Ontario plot (Table 1). Greater numbers of Fusarium taxa were isolated from nonrhizosphere soil samples than from roots in both the Wisconsin nursery and the nonfumigated Ontario plots. Rhizosphere soil samples yielded fewer taxa from fumigated Ontario plots relative to the number of taxa isolated from symptomatic roots, whereas the number of taxa was greater from rhizosphere samples than from lesion samples of seedlings grown in the Wisconsin nursery as well as in the nonfumigated plots located in Ontario. Fusarium oxysporum var. redolens, F. oxysporum, and F. solani were isolated from all three sources in both locations; F. acuminatum and F. eguiseti were obtained from the two soil environments in both locations and differed in occurrence in root lesions by location; F. sporotrichioides and F.

OCAMB, JUZWIK: WHITE PINEFUSARIUM

327

Table 1. Fu~ariutnspecies isolated from root lesions on 2-year-old eastern white pine seedlings. rhizosphere soil, and nonrhizosphere soil in two bareroot nurseries Root lesions Fcisurium species F. acuminatutn F. eguiseti F. graminearurn F. ox-ysporum F. o,xysporum var. redolens F. prol$eratum F. sambucinum F. soluni F. spomtr-iclzioides

WI$

+ + + +

+ +

Ontariofum.

Rhizosphere soil

Ontariononfum.

+ + +

+ +

+ +

+

WI

Ontariofum.

+ + + + + +

Nonrhizosphere soil

Ontariononfum.

+ + +

+ +

+ + +

WI

Ontario- Ontariofum. nonfum.

+ + + + + + + +

+ +

+ +

+ +

+ +

+

+ +

$WI is a fumigated field in F.G. Wilson State Nursery, Boscobel. Wisconsin, USA; Ontario-fum. is a fumigated section in St. Williams Provincial Nursery, St. Williams, Ontario, Canada; and Ontario-nonfum. is a nonfumigated section in St. Williams Provincial Nursery, St. Williams, Ontario, Canada.

proliferaturn were recovered from all three sources in the Wisconsin field; F. graminearurn was isolated from the two soil environments only in the Wisconsin field; and F. sambucinunz was recovered from root lesions and nonrhizosphere soil only in the Ontario field. The subgroup F. oxysporurn var. redolens (Booth 1971, Gerlach & Nirenberg 1982) was delineated on carnation leaf agar, due to its prevalence, even though its status as a separate taxon is not universally accepted (Messiaen & Cassini 1981). Not all plots yielded symptomatic seedlings in our samples. Generally, 50% of plots contained seedlings with root rot on the first sample date whereas nearly 90% of the plots yielded seedlings with diseased roots on the last sample date (Table 2). Fusariurn species were not recovered frorn every seedling that exhibited root necrosis. Recovery was 30% to 98% of affected seedlings collected on the different dates. Prevalence of Fusarium species in root lesions. Fusarium oxysporum var. redolens was the taxon most frequently associated with root lesions in the Wisconsin field, followed by F. proliferatum (Fig. I). Fusariutn oxysporum var. redolens, F. oxysporum, and F. solani were the most frequently isolated species from seedlings in both sections of the Ontario field. There was no qualitative difference in the Fusarium species isolated from both sections, but the number of seedlings with root lesions from which Fusarium species were obtained was greater in the nonfumigated section than in the fumigated section for the July and October sampling dates. The number of seedlings with root lesions that yielded F~rsarium species increased throughout the growing season at both nurseries. Mixtures involving seven different pairs of Fziscrriurn species were associated with root lesions on affected seedlings from both nurseries. Fusarium species mixtures were less prevalent than the predominant two or three Fusarium species isolated from root lesions in all sites and dates with one

exception. In the October samples from the nonfumigated section of the Ontario field, different mixtures of Fusarium species were recovered from diseased seedling roots (Fig. 1): F. oxysporum and F. oxysporum var. redolens from seven seedlings, F. oxysporum var. redolens and F. solani frorn six, and F. oxysporurn and F. solani frorn three seedlings. Prevalence of Fusarium species in nonrhizosphere soil. Mean number of colony-forming units (cfu) of Fusariunz species per gram of soil peaked in all fields in July: 3779 in the Ontario-nonfumigated field; 2695 in the Ontario-fumigated field; and 467 1 in the Wisconsin field. The predominant Fusarium species in each field differed slightly by sampling date. In the Wisconsin field, F. oxysporum var. redolens was the taxon most frequently recovered in April, but F. proliferatum, present at low levels in April, was the predominant species in July and October (Fig. 2A). In the Ontario-fumigated field, only F. oxysporum var. redolens and F. sambucinum were found in April; F. oxysporum appeared in July Table 2. Incidence of fusarium root rot of 2-year-old eastern white pine seedlings by plot and seedling basis in an Ontario and a Wisconsin bareroot nursery Nursery fieldWisconsin-fumigated Ontarlo-fumigated Ontario-nonfurnigated

Sampling date

Affected plots (%)

14 May 199 1 21 Jul 1991 8 Oct I991 30 Apr 199 1 2 Jul 1991 2 Oct 1991 30 Apr 199 1 2 Jul 1991 2 Oct 1991

42 79 88 50 67 92 50 100 92

Affected seedlings (5%)

$Wisconsin-fumigated is located in F.G. Wilson State Nursery, Boscobel, Wisconsin, USA and Ontario-fumigated and -nonfumigated plots are located in St. Williams Provincial Nursery, St. Williams, Ontario. Canada.

328 CANADIAN JOURNAL OF PLANT PATHOLOGY, VOLUME 17,1995

and was the most prevalent species in October (Fig. 2B). Flrsariurn o.xyspontm var. redolerzs and F. oxysporunz were frequently recovered on two of the sampling dates in both the fumigated and nonfumigated sections of the Ontario field; however, F. solani was present throughout the season in the nonfumigated section but appeared only during the October sampling date in the fumigated section (Fig. 2B & 2C). Prevalence of Fusarium species in rhizosphere soil. The mean number of colony-forming units encompassing all F~tsariumspecies (cfu per gram of soil) in both sections of the Ontario field peaked in July. Data are not available to make a similar comparison in the Wisconsin field. F. oxysporurn var. redolens and F. oxysporurn were the predominant taxa in rhizosphere soil in both sections of the Ontario field on all three sampling dates and in the Wisconsin field in October (Fig. 2). However, F. solani was found at much higher frequencies on all

three dates in the rhizosphere soil from the nonfumigated section of the Ontario field than in the fumigated section. F~isnriurnproliferaturn predominated in the Wisconsin field rhizosphere soil in May, but the number of colonies of F: o,rj\;.sporurn,F. oxysporunz var. redolens, and F. solani reached counts similar to those of F. proliferaturn by October.

Discussion Three Fusariurn species were consistently associated with diseased roots of eastern white pine seedlings in our studies. These findings augment previous reports that F. oxysporurn is associated with white pine root rot (Enebak 1988, Honhart & Juzwik 1988). Although we isolated eight species of Fusurium, F. oxysporum, F. proliferuturn, and F. solani were the most prevalent on roots and in rhizosphere and nonrhizosphere soils. Also, we found F. oxysporum var. redolens to be consistently associated with root

Wisconsin-fumigated

F. acuminatum

A

F. oxysporum var. redoiens

F. oxysporum var redolens

F. rolani Fusarium mixtures

F solani

0

2

4 6 8 10 12 Root diseased seedlings yielding Fusarium species (%)

14

Figure 1. Fuscrriurn species isolated from root lesions on 2-year-old eastern white pine seedlings in two bareroot nurseries. The Wisconsin-fumigated field is located in F. G. Wilson State Nursery, Boscobel. Wisconsin, USA. Ontario-fumigated and Ontario-nonfumigated are half sections of one field in St. Williams Provincial Nursery, St. Williams, Ontario, Canada. Samples were collected on 14 May, 21 July, and 8 October 1991 from the Wisconsin nursery. Samples were collected on 30 April, 2 July, and 2 October 1991 from the Ontario nursery.

OCAMB, JUZWIK: WHITE PINEFUSARIUM

ar) F. equiseti

i

October

329

1 I

F. oxysporum var. redclens

F. profiferaturn F. sporotrichioides

//

I

I

35

30

2515 10 NO. cfulg dry soil (x lo2)

5

0

20

40

60

80100 1700 NO. cfu/g dry soil jx l o 2 )

1800

F. equiseti

I

F. oxysporvm

. I sporvm var. redolsns

1

F. solani

I

I

I

15 10 5 NO. cfulg dry soil (x lo2)

0

5

I 20

I

10 20 No. cfulg dry so11 (x lo4)

25

Ontario-nonfumigated

4

Nonrhizosphere

F. acumhatum

Rhizosphere

F. oxysporum

30

25

20 15 10 NO. cfulg dry soil (x lo2)

5

0

5

10 15 No. cfulg dry soil

20

25

(x lo4)

Figure 2. Fusarium species isolated from nonrhizosphere and rhizosphere soil collected from two eastern white pine bareroot nursery fields. The Wisconsin-fumigated field is located in F.G. Wilson State Nursery, Boscobel, Wisconsin, USA. Ontario-fumigated and Ontario-nonfumigated plots are half sections of one field in St. Williams Provincial Nursery, St. Williams, Ontario, Canada. Samples were collected on 14 May, 21 July, and 8 October 1991 from the Wisconsin nursery. Samples were collected on 30 April, 2 July, and 2 October 1991 from the Ontario nursery.

lesions in our study, confirming previous reports that a subgroup of F. oxysporum may play a significant role in root rot of conifers (Booth 1971, Matuo & Ghiba 1966). Mixtures of Fusarium species were isolated from root lesions, but such mixtures were isolated less often than were single species of F~isarittm. This suggests that, compared to single species of Fztsarium, mixtures may be less important contributors to fusarium root rot of eastern white pine. Riffle and Strong (1960) reported that F. moniliforme and F. solarzi were implicated in root rot of white pine, and our data support their conclusion. Since Riffle and Strong's report, F. p~.olifet-aturnhas

been separated from F. moniliforme (Nirenberg 1976). Riffle and Strong may have reported F. pt-oliferatum had they known of the existence of this species. This is the first report of F. acurninatum, F. equiseti, F. sambucin~tm,and F. sporotrichioides associated with diseased roots of eastern white pine seedlings. However, the low rate of isolation of these four species from symptomatic roots raises the question of whether these species are secondary rather than primary colonizers of white pine roots. The species composition of fisariurn assemblages in white pine nursery fields apparently varies with season, geographic location, and management practices,

330 CANADIAN JOURNAL OF PLANT PATHOLOGY. VOLUME 17. 1995

among other factors. Incidence of symptomatic seedlings that yielded Fusariz~misolates increased as the growing season progressed; total number of colony-foming units of all Fusuriztm species, in rhizosphere and nonrhizosphere soil. peaked in July. Generally, the number of species of Fusnrium detected was greater in October than in AprilMay. However. this may be attributed to secondary colonization by these minor species or increases in specific populations that enabled detection of the minor Fusnriunz species rather than migration of these minor species into the fumigated fields. The Wisconsin site yielded a greater diversity of Fusariunz species from rhizosphere and nonrhizosphere samples than did the Ontario location. In addition, a predominant Fusuriurn species in the Wisconsin nursery, F. proliferaturn, was not detected at the Ontario nursery. A lessor diversity of Fusarium species was associated with diseased seedlings in nonfumigated plots than that found in fumigated plots. Fumigation frequency, type of fumigant, soil characteristics, and site characteristics may contribute to the diversity of Fusariurn species present. These factors may influence when, where, how, and which Fusan'urzz species recolonize fumigated soil Many researchers have observed that if a single species is prevalent, that species is usually the pathogen and it competes successfully against other less pathogenic or saprophytic species (Bruehl 1987). Conversely, a single species or a combination of species occupying the rhizosphere may mediate root invasion by a pathogen, especially if the pathogen has a low competitive saprophytic ability demonstrated by its inability to colonize the rhizosphere (Ocamb & Kommedahl 1994). The role of each F u s a r i z ~ m species has yet to be resolved, and pathogenicity tests have been initiated to ascertain the primary cause of white pine root rot. Moreover, site and seasonal influences need to be determined in order to understand white pine root rot. Nevertheless, it is possible that multiple Fusarizinz taxa cause root rot of white pine in bareroot nurseries, and effective disease control measures are needed to produce healthy seedlings. The technical assistance of J. Gocool, M. Lo, J. Rykhus, K. Stolhammer, and V. Tran is gratefully acknowledged. This work was funded in part by the Wisconsin Department of Natural Resources and by an Ontarlo Renewable Resources Research Grant. Mention of trade names does not constitute endorsement by the U.S. Department of Agriculture.

Booth, C. 1971. The Genus Furari~r-/?. Commonw. Mycol. Instit., Kew. Surrey. England. 237 pp.

Bruehl, G.W. 1987. Soilborne Plant Pathogens. Macmillan Publishing Company, New York. 368 pp.

Dhingra, O.D., and J.B. Sinclair. 1985. Basic Plant Pathology Methods. CRC Press, Boca Raton, Florida. 355 pp.

Enebak, S.A. 1988 Control of- Soilborne Pathogens In Forest Tree Nursenet; M.S The515 Cni.~ersityof- Minnesota. St Paul, MN 55 PP Gerlach, W., and H. Nirenberg. 1982 The Genus Fztr~trurtn- a P~ctorialAtlas Mltt Biol Bundesanst. Land-Forstwlrtsch Berlin-Dahlem 406 pp Greifenhagen, S., G. Halicki, and T. Meyer. 1990 Peit management In nursery stock production 1989 - Annual Progre\s Report. Fore5t Health and Protection Section, Ont Mln. Nat. Res., Sault Ste. Marie, Ontarlo, Canada. Honhart, C., and J. Juzwik. 1988. The impact of Fu\artum, Cylindrocarpon and Cyl~ndroclad~um root rot on whlte plne seedlings In compartment A59, Mldhurst Nursery, 1987 - Pest Control Sec. Rep. PC-32. Pest Control Sectlon, Ont Min. Nat. Res., Sault Ste. Marte, Ontano, Canada 8 pp. Kommedahl, T., C.E. Windels, and R.E. Stucker. 1979. Occurrence of Furanurn specie\ In roots and stalks of symptomless corn plant\ during the growlng season. Phytopathology 69:961-966. Matuo, T., and 0. Chiba. 1966. Species and formae speciales of Fusarla c a u s ~ n gdamping-off and root rot of coniferous seedllngs In Japan. Ann. Phytopath. Soc. Japan 32: 14-22. Messiaen, C.M., and R. Cassini. 1981 Taxonomy of Furanunz. Page\ 427-445 In P.E. Nelson, T.A. Toussoun, and R.J. Cook, eds., Fusarrum: Disease?, Blology, and Taxonomy. The Pennsylvan~aUniversity Pre\s, Univers~tyPark, Pennsylvanla, USA. Nash, S.M., and W.C. Snyder. 1962. Quanrltatlve estimations by plate couilts of propagules of the bean root rot Furarluin In field soil\. Phytopathology 52567-572. Nelson, P.E., T.A. Toussoun, and W.F.O. Marasas. 1983. Fzlsarzuin Spec~e\.An Illustrated Guide for Identlf~catron.The P e n n \ y l v a n ~ aState University Pres\. Unlverslty Park, Pennsylvctnla, USA. 193 pp. Nirenberg, H. 1976. Undersuchungen uber die morpholog~sche und biologische Dlfferenz~erungIn der Fu\arlum-Sektion Liqeola. Mitt. Blol. Bundeqan\t. Land-Forstwlrtsch. BerllnDahlem 169:1-1 17 (original not read). Ocamb, C.M., and T. Kommedahl. 1994. Rh~zospherecompetence of Fusartunz species c o l o n l z ~ n g corn roots. Phytopathology 84.166-1 72. Papavizas, G.C. 1967. Evaluation of various medla and antlmicrobial agents for lsolatlon of Fusurrum from sod. Phytopathology 57:848-852. Prey, A.J., D.J. Hall, and J.E. Gummings. 1983. Forest Pest Conditions In Wisconsin, Annual Report. Department of Natural Resources. Dlv. of Resource Management, Bureau of Forestry. 27 PP' Prey, A.J., D.J. Hall, and J.E. Cummings. 1985. F o r s t Pest Conditions in Wlsconsln. Annual Report. Department of Natural Resource\, Dlv. of Resource Management, Bureau of Forestry. 44 PP. Prey, A.J., D.J. Hall, and J.E. Cummings. 1986 Forest Pest Condltlons In Wlsconsin, Annual Report. Department of Natural Resources. Dlv. of Resource Management, Bureau ot Forestry. 41 PP. Renlund, D.W. 1980. Forest Pest Conditions In Wlsconsin, Annual Report. Department of Natural Resources, Div of Resource Management, Bureau of Forestry. 37 pp. Renlund, D.W. 1981 Forest Pest Cond~tlonsin Wisconsin, Annual Report Department of Natural Resources, DIV of Resource Management. Bureau of Forestry 32 pp Riffle, J.W., and F.C. Strong. 1960. Studies rn white plne seedling root rot Qudrterly Bull Michlgan Agr~c Expt. Sta. 42.545-853 Wenner, N., and W. Merrill, 1984 New conifer hosts for Fu,nrzitn? root rot in Pennsylvanla. Plant Dls. 68:536. Windels, C.E., P.M. Burnes, and T. Kommedahl. 1988 Fiveyear preiervation of Futnrtrtrn species on illica gel and 5011. Phytopathology 78.107-109